A projection photolithography tool is an equipment for projecting a pattern on a photomask onto the surface of a wafer using a projection objective. During exposure carried out by such a photolithography tool, the wafer may be out of focus or tilt relative to a focal plane of the projection objective due to thickness inconsistencies or surface shape irregularities of the wafer or changes in accuracy or reproducibility of the focal plane. As a result, some areas of the wafer surface in the exposure field of view may not be within an effective depth of focus, and the photolithography quality can be seriously impaired. For this reason, focusing and leveling is necessary for precise positional control over the wafer within the exposure field of view, and the measurement accuracy of the used measurement device for focusing and leveling plays an extremely critical role. With the requirements for photolithography processes continuing to increase, the measurement accuracy of such measurement devices for focusing and leveling must be continuously improved accordingly. Measurement stability is one of the several factors determining the measurement accuracy, including not only those related to the environment, such as temperature and pressure, but also a drift of a used detector incurred by its power consumption and the like.
As shown in FIG. 1, in a conventional measurement device for focusing and leveling, light from a light source 1 propagates through an illumination lens group 2, slits 3 and a projection lens group 4 and is incident on a wafer 5. The reflected light from the wafer 5 passes through a detecting lens group 6 and is incident on a linear charge coupled device (CCD) in a detector 7, creating a light spot thereon. Monitoring positional variations of the spot on the linear CCD enables calculation of a defocus amount of the wafer 5. This technique, however, neither takes into account the influence of a drift of the detector 7 itself on the measurement result, nor does it monitor the drift of the detector 7, despite the fact that the drift of the detector 7 may cause deviation in the measurement result, deteriorate the device's measurement accuracy and adversely affect its measurement stability.